Fuel for use in a fuel cell system

ABSTRACT

A fuel for a fuel cell system comprises wherein said fuel comprises hydrocarbons comprising 60 mol. % or more of saturates, 40 mol. % or less of olefins, 0.5 mol. % or less of butadiene, 0.1 mol. % or more of isoparaffin in saturates having carbon atoms of 4 or more and being a gaseous phase under normal temperature and pressure. The fuel for a fuel cell system has a high power generation quantity per weight, a high power generation quantity per CO 2  emission, a low fuel consumption, a small evaporative gas (evapo-emission), small deterioration of a fuel cell system comprising such as a reforming catalyst, a water gas shift reaction catalyst, a carbon monoxide removal catalyst, fuel cell stacks and the like to maintain the initial performances for a long duration, good handling properties in view of storage stability and inflammability, and a low preheating heat quantity.

TECHNICAL FIELD

[0001] The present invention relates to a fuel to be used for a fuel cell system.

BACKGROUND ART

[0002] Recently, with increasing awareness of the critical situation of future global environments, it has been highly expected to develop an energy supply system harmless to the global environments. Especially urgently required are to reduce CO₂ to prevent global warming and reduce harmful emissions such as THC (unreacted hydrocarbons in an exhaust gas), NO_(X), PM (particulate matter in an exhaust gas: soot, unburned high boiling point and high molecular weight fuel and lubricating oil). Practical examples of such a system are an automotive power system to replace a conventional Otto/Diesel engine and a power generation system to replace thermal power generation.

[0003] Hence, a fuel cell, which has high energy efficiency and emits only H₂O and CO₂, has been regarded as a most expectative system to response to respond to social requests. In order to achieve such a system, it is necessary to develop not only the hardware but also the optimum fuel.

[0004] Conventionally, as a fuel for a fuel cell system, hydrogen, methanol, and hydrocarbons have been candidates.

[0005] As a fuel for a fuel cell system, there is methanol except for hydrogen. Methanol is advantageous in a point that it is relatively easy to reform, however power generation quantity per weight is low and owing to its toxicity, handling has to be careful. Further, it has a corrosive property, special facilities are required for its storage and supply.

[0006] Like this, a fuel to sufficiently utilize the performances of a fuel cell system has not yet been developed. Especially, as a fuel for a fuel cell system, the following are required: power generation quantity per weight is high; power generation quantity per CO₂ emission is high; a fuel consumption is low in a fuel cell system as a whole; an evaporative gas (evapo-emission) is a little; deterioration of a fuel cell system comprising such as a reforming catalyst, a water gas shift reaction catalyst, a carbon monoxide conversion catalyst, fuel cell stacks and the like is scarce to keep the initial performances for a long duration; a starting time for the system is short; and storage stability and handling easiness are excellent.

[0007] Incidentally, in a fuel cell system, it is required to keep a fuel and a reforming catalyst at a proper temperature, the net power generation quantity of the entire fuel cell system is equivalent to the value calculated by subtracting the energy necessary for keeping the temperature (the energy for keeping balance endothermic and exothermic reaction following the preheating energy) from the actual power generation quantity. Consequently, if the temperature for the reforming is lower, the energy for preheating is low and that is therefore advantageous and further the system starting time is advantageously shortened. In addition, it is also necessary that the energy for preheating per fuel weight is low. If the preheating is insufficient, unreacted hydrocarbon (THC) in an exhaust gas increases and it results in not only decrease of the power generation quantity per weight but also possibility of becoming causes of air pollution. To say conversely, when some kind of fuels are reformed by the same reformer and the same temperature, it is more advantageous that THC in an exhaust gas is lower and the conversion efficiency to hydrogen is higher.

[0008] The present invention, taking such situation into consideration, aims to provide a fuel suitable for a fuel cell system satisfying the above-described requirements in good balance.

DISCLOSURE OF THE INVENTION

[0009] Inventors of the present invention have extensively investigated to solve the above-described problems and found that a fuel comprising hydrocarbons with specific compositions is suitable for a fuel cell system.

[0010] That is, the fuel for a fuel cell system according to the present invention comprises:

[0011] (1) hydrocarbons comprising 60 mol. % or more of saturates, 40 mol. % or less of olefins, 0.5 mol. % or less of butadiene, 0.1 mol. % or more of isoparaffin in saturates having carbon atoms of 4 or more and being a gaseous phase under normal temperature and pressure.

[0012] The fuel comprising hydrocarbons with the above-described compositions is preferable to satisfy the following additional requirements;

[0013] (2) a sulfur content is 50 ppm by mass or less;

[0014] (3) a content of hydrocarbons having carbon numbers of 2 or less is 5 mol. % or less, a content of hydrocarbons having carbon numbers of 3 and 4 in total is 90 mol. % or more, a content of hydrocarbons having carbon numbers of 5 or more is 5 mol. % or less;

[0015] (4) vapor pressure at 40° C. is 1.55 MPa or less;

[0016] (5) density at 15° C. is 0.500 to 0.620 g/cm³;

[0017] (6) corrosiveness to copper at 40° C. for 1 hour is 1 or less;

[0018] (7) heat capacity of the fuel is 1.7 kJ/kg° C. or less at 15° C. in gaseous phase.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 shows a flow chart of a steam reforming type fuel cell system employed for evaluation of a fuel for a fuel cell system of the invention.

[0020]FIG. 2 is a flow chart of a partial oxidation type fuel cell system employed for evaluation of a fuel for a fuel cell system of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] Hereinafter, the contents of the invention will be described further in detail.

[0022] In the present invention, the hydrocarbons with specific compositions are ones comprising 60 mol. % or more of saturates (M(S)), 40 mol. % or less of olefins (M(O)), 0.5 mol. % or less of butadiene (M(B)), 0.1 mol. % or more of isoparaffin (M(IP)) in saturates having carbon atoms of 4 or more, and are a gaseous phase under normal temperature and pressure.

[0023] The saturates (M(S)) is 60 mol. % or more, preferably 80 mol. % or more, more preferably 95 mol. % or more and most preferably 99 mol. % or more in view of a high power generation quantity per weight, a high power generation quantity per CO₂ emission, a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, and the like.

[0024] The olefins (M(O)) is 40 mol. % or less, preferably 10 mol. % or less and most preferably 1 mol. % or less in view of a high power generation quantity per weight, a high power generation quantity per CO₂ emission, a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, small deterioration of a reforming catalyst to maintain the initial performances for a long duration, a good storage stability, and the like.

[0025] The butadiene (M(B)) is 0.5 mol. % or less and preferably 0.1 mol. % or less in view of a high power generation quantity per weight, a high power generation quantity per CO₂ emission, a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, small deterioration of a reforming catalyst to maintain the initial performances for a long duration, a good storage stability, and the like.

[0026] The isoparaffin (M(IP)) in saturates having carbon atoms of 4 or more is 0.1 mol. % or more, preferably 1 mol. % or more, more preferably 10 mol. % or more, furthermore preferably 20 mol. % or more and most preferably 30 mol. % or more in view of a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, and the like.

[0027] Incidentally, the above-described (M(S)), (M(O)), (M(B)) and (M(IP)) are values measured by JIS K 2240, “Liquefied Petroleum Gases 5.9 Methods for Chemical Composition Analysis”.

[0028] Further, the content of sulfur in a fuel of the invention is not particularly restricted, however, because deterioration of a fuel cell system comprising such as a reforming catalyst, a water gas shift reaction catalyst, a carbon monoxide removal catalyst, fuel cell stacks, and the like can be suppressed to low and the initial performances can be maintained for a long duration, the content is preferably 50 ppm by mass or less, more preferably 10 ppm by mass or less, further more preferably 1 ppm by mass or less.

[0029] Then, it is most preferably to satisfy the above-described preferable ranges of sulfur and the above-described preferable ranges of compositions since deterioration of a fuel cell system comprising such as a reforming catalyst, a water gas shift reaction catalyst, a carbon monoxide removal catalyst, fuel cell stacks, and the like can be suppressed to low and the initial performances can be maintained for a long duration.

[0030] Here, sulfur content means that measured by JIS K 2240, “Liquefied Petroleum Gases 5.5 or 5.6 Determination of sulfur content”.

[0031] In the fuel according to the invention, compositions of respective carbon atoms are not particularly restricted, however, it is preferable that a content of hydrocarbons having carbon numbers of 2 or less is 5 mol. % or less, a content of hydrocarbons having carbon numbers of 3 and 4 in total is 90 mol. % or more, and a content of hydrocarbons having carbon numbers of 5 or more is 5 mol. % or less.

[0032] The content of hydrocarbons having carbon numbers of 2 or less is preferably 5 mol. % or less and more preferably 3 mol. % or less in relation to the storage, inflammability and evapo-emission and the like. The content of hydrocarbons having carbon numbers of 3 and 4 in total is 90 mol. % or more and more preferably 95 mol. % or more in view of a high power generation quantity per weight, a high power generation quantity per CO₂ emission, a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, small deterioration of a reforming catalyst to maintain the initial performances for a long duration, and the like. The content of hydrocarbons having carbon numbers of 5 or more is 5 mol. % or less and more preferably 2 mol. % or less in view of a high power generation quantity per weight, a high power generation quantity per CO₂ emission, a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, small deterioration of a reforming catalyst to maintain the initial performances for a long duration, and the like.

[0033] Incidentally, the compositions of respective carbon atoms mentioned above are values measured by JIS K 2240, “Liquefied Petroleum Gases 5.9 Methods for Chemical Composition Analysis”.

[0034] Further, vapor pressure of a fuel of the invention is not particularly restricted, however, it is preferably 1.55 MPa or less and more preferably 1.53 MPa or less at 40° C. in relation to the storage, inflammability and evapo-emission and the like.

[0035] Incidentally, the vapor pressure at 40° C. is measured by JIS K 2240, “Liquefied Petroleum Gases 5.4 Calculation method for density and vapor pressure”.

[0036] Further, density of a fuel of the invention is not particularly restricted, however, it is preferably 0.620 g/cm³ or less at 15° C. in view of a high power generation quantity per weight, a high power generation quantity per CO₂ emission, a low fuel consumption of a fuel cell system as a whole, a low THC in an exhaust gas, short starting time of a system, small deterioration of a reforming catalyst to maintain the initial performances for a long duration, and the like, and more preferably 0.500 g/cm³ or less to exhibit the effects of the invention.

[0037] Incidentally, the density at 15° C. is measured by JIS K 2240, “Liquefied Petroleum Gases 5.7 or 5.8 Calculation method for density and vapor pressure”.

[0038] Further, the corrosiveness to copper of a fuel according to the invention is not particularly restricted, however, the corrosiveness thereof is preferable to 1 or less at 40° C. for 1 hour because deterioration of a fuel cell system comprising such as a reforming catalyst, a water gas shift reaction catalyst, a carbon monoxide removal catalyst, fuel cell stacks, and the like can be suppressed to low and the initial performances can be maintained for a long duration.

[0039] Incidentally, the corrosiveness to copper at 40° C. for 1 hour is measured by JIS K 2240, “Liquefied Petroleum Gases 5.10 Corrosiveness to copper”.

[0040] Further, in the invention, heat capacity of a fuel is not particularly restricted, however, the heat capacity is preferably 1.7 kJ/kg·° C. or less at 15° C. and in gaseous phase in view of a low fuel consumption of a fuel cell system as a whole.

[0041] The heat capacity is measured by means of calorimeters such as water calorimeter, ice calorimeter, vacuum calorimeter, adiabatic calorimeter and the like.

[0042] A production method of the fuel of the invention is not particularly restricted. As a practical method, for example, the fuel can be prepared by blending one or more of the following hydrocarbon base materials; a straight-run propane fraction containing propane as a main component obtained by treating heavy oils with a distillation apparatus, naphtha reforming apparatus and the like, a straight-run desulfurized propane fraction obtained by desulfurizing the straight-run propane fraction, a straight-run butane fraction containing butane as a main component obtained by treating heavy oils with a distillation apparatus, naphtha reforming apparatus, alkylation apparatus and the like, a straight-run desulfurized butane fraction obtained by desulfurizing the straight-run butane fraction, a cracked propane fraction containing propane and propylene as main components obtained by cracking heavy oils with a fluid catalytic cracking apparatus (FCC) and the like, a cracked butane fraction containing butane and butene as main components obtained by treating heavy oils with a fluid catalytic cracking apparatus (FCC) and the like.

[0043] Among them, preferable materials as the base materials for the production of the fuel of the invention are the straight-run desulfurized propane fraction, the straight-run desulfurized butane fraction and the like.

[0044] A fuel of the invention is to be employed as a fuel for a fuel cell system. A fuel cell system mentioned herein comprises a reformer for a fuel, a carbon monoxide conversion apparatus, fuel cells and the like, however, a fuel of the invention may be suitable for any fuel cell system.

[0045] The reformer is an apparatus for obtaining hydrogen, by reforming a fuel. Practical examples of the reformer are:

[0046] (1) a steam reforming type reformer for obtaining products of mainly hydrogen by treating a heated and vaporized fuel and steam with a catalyst such as copper, nickel, platinum, ruthenium and the like;

[0047] (2) a partial oxidation type reformer for obtaining products of mainly hydrogen by treating a heated and vaporized fuel and air with or without a catalyst such as copper, nickel, platinum, ruthenium and the like; and

[0048] (3) an auto thermal reforming type reformer for obtaining products of mainly hydrogen by treating a heated and vaporized fuel, steam and air, which carries out the partial oxidation of (2) in the prior stage and carries out the steam type reforming of (1) in the posterior stage while using the generated heat of the partial oxidation reaction with a catalyst such as copper, nickel, platinum, ruthenium and the like.

[0049] The carbon monoxide conversion apparatus is an apparatus for removing carbon monoxide which is contained in a gas produced by the above-described reformer and becomes a catalyst poison in a fuel cell and practical examples thereof are:

[0050] (1) a water gas shift reactor for obtaining carbon dioxide and hydrogen as products from carbon monoxide and steam by reacting a reformed gas and steam in the presence of a catalyst of such as copper, nickel, platinum, ruthenium and the like; and

[0051] (2) a preferential oxidation reactor for converting carbon monoxide into carbon dioxide by reacting a reformed gas and compressed air in the presence of a catalyst of such as platinum, ruthenium and the like, and these are used singly or jointly.

[0052] As a fuel cell, practical examples are a proton exchange membrane type fuel cell (PEFC), a phosphoric acid type fuel cell (PAFC), a molten carbonate type fuel cell (MCFC), a solid oxide type fell cell (SOFC) and the like.

[0053] Further, the above-described fuel cell system can be employed for an electric automobile, a hybrid automobile comprising a conventional engine and electric power, a portable power source, a dispersion type power source, a power source for domestic use, a cogeneration system and the like.

EXAMPLES

[0054] The properties of base materials (LPG) employed for the respective fuels for examples and comparative examples are shown in Table 1.

[0055] Also, the compositions and properties of the respective fuels employed for examples and comparative examples are shown in Table 2. TABLE 1 straight-run straight-run desulfurized FCC-C3 FCC-C4 C3 fraction C4 fraction fraction fraction *1 *2 *3 *4 DME *5 sulfur mass ppm 7 <1 5 34 <1 density @ 15° C. 0.509 0.577 0.518 0.591 0.600 vapor pressure @ 40° C. Mpa 1.33 0.34 1.50 0.39 0.88 corrosiveness to copper 1a 1a 1a 1 — carbon number: C²⁻ mol. % 2.5 0.0 0.0 0.0 — carbon number: C₃ mol. % 96.6 0.0 99.8 2.4 — carbon number: C₄ mol. % 0.9 99.9 0.2 92.4 — carbon number: C₅₊ mol. % 0.0 0.1 0.0 5.2 — saturates mol. % 99.9 99.9 19.7 53.9 — olefins mol. % 0.1 0.1 80.3 46.1 — butadiene mol. % 0.0 0.0 0.0 0.2 — isoparaffines in mol. % 78.2 35.8 100.0 81.4 — saturates having carbon numbers of 4 or more

[0056] TABLE 2 Ex. 1 Ex. 2 Ex. 3 Comp. 1 Comp. 2 Mixing ratio (vol. %) straight-run C3 fraction 100 25 straight-run desulfurized C4 fraction 100 75 FCC-C3 fraction 94 FCC-C4 fraction 100 ethane 6 Analytical resuits of properties sulfur mass ppm 7 <1 2 5 34 density g/cm³ 0.509 0.577 0.560 0.508 0.591 vapor pressure Mpa 1.33 0.34 0.60 1.75 0.36 distribution of carbon carbon number: C²⁻ mol. % 2.5 0.0 0.7 5.8 0.0 numbers (hydrocarbon carbon number: C₃ mol. % 96.6 0.0 27.1 94.0 2.4 moieties) carbon number: C₄ mol. % 0.9 99.9 72.1 0.2 92.4 carbon number: C₅₊ mol. % 0.0 0.1 0.1 0.0 5.2 composition saturates mol. % 99.9 99.9 99.9 24.4 53.9 olefins mol. % 0.1 0.1 0.1 75.6 46.1 butadiene mol. % 0.0 0.0 0.0 0.0 0.2 isoparaffines in saturates having mol. % 78.2 35.8 35.9 100.0 80.6 carbon numbers of 4 or more corrosiveness to copper 1a 1a 1a 1a 1 net heat of combustion kJ/kg 46330 45670 45820 45930 45440 heat capacity gas kJ/kg · ° C. 1.62 1.62 1.62 1.52 1.55

[0057] These respective fuels were subjected to evaluation tests for a fuel cell system.

[0058] Fuel Cell System Evaluation Test

[0059] (1) Steam Reforming

[0060] A fuel and water were evaporated by electric heating and led to a reformer filled with a noble metal type catalyst and kept at a prescribed temperature by an electric heater to generate a reformed gas enriched with hydrogen.

[0061] The temperature of the reformer was adjusted to be the minimum temperature (the minimum temperature at which no THC was contained in a reformed gas) at which reforming was completely carried out in an initial stage of the test.

[0062] Together with steam, a reformed gas was led to a carbon monoxide conversion apparatus (a water gas shift reaction) to convert carbon monoxide in the reformed gas to carbon dioxide and then the produced gas was led to a solid polymer type fuel cell to carry out power generation.

[0063] A flow chart of a steam reforming type fuel cell system employed for the evaluation was illustrated in FIG. 1.

[0064] (2) Partial Oxidation

[0065] A fuel is evaporated by electric heating and together with air, the evaporated fuel was led to a reformer filled with a noble metal type catalyst and kept at a 1100° C. by an electric heater to generate a reformed gas enriched with hydrogen.

[0066] Together with steam, a reformed gas was led to a carbon monoxide conversion apparatus (a water gas shift reaction) to convert carbon monoxide in the reformed gas to carbon dioxide and then the produced gas was led to a solid polymer type fuel cell to carry out power generation.

[0067] A flow chart of a partial oxidation type fuel cell system employed for the evaluation was illustrated in FIG. 2.

[0068] (3) Evaluation Method

[0069] The amounts of H₂, CO, CO₂ and THC in the reformed gas generated from a reformer were measured immediately after starting of the evaluation test. Similarly, the amounts of H₂, CO, CO₂ and THC in the reformed gas generated from a carbon monoxide conversion apparatus were measured immediately after starting of the evaluation test.

[0070] The power generation quantity, the fuel consumption, and the CO₂ amount emitted out of a fuel cell were measured immediately after starting of the evaluation test and 100 hours later from the starting.

[0071] The energy (preheating quantities) necessary to heat the respective fuels to a prescribed reforming temperature were calculated from the heat capacities and the heat of vaporization.

[0072] Further, these measured values, calculated values and the heating values of respective fuels were employed for calculation of the performance deterioration ratio of a reforming catalyst (the power generation amount after 100 hours later from the starting divided by the power generation amount immediately after the starting), the thermal efficiency (the power generation amount immediately after the starting divided by the net heat of combustion of a fuel), and the preheating energy ratio (preheating energy divided by the power generation amount).

[0073] The respective measured values and the calculated values are shown in Table 3. TABLE 3 Ex. 1 Ex. 2 Ex. 3 Comp. 1 Comp. 2 Evaluation results Electric power generation by steam reforming method (reforming temperature = optimum reforming temperature 1)) Optimum reforming ° C. 650 640 640 680 670 temperature Electric energy kJ/fuel kg initial performance 31140 30700 30800 29180 29460 100 hours later 31110 30690 30780 28700 28520 performance deterioration ratio 100 hours later 0.10% 0.03% 0.06% 1.64% 3.19% Thermal efficiency 2) initial performance 67% 67% 67% 64% 65% CO₂ generation kg/fuel kg initial performance 2.993 3.029 3.021 3.102 3.079 Energy per CO₂ KJ/CO₂-kg initial performance 10404 10135 10195 9407 9568 Preheating energy 3) kJ/fuel kg 1010 1000 1000 1000 1000 Preheating energy ratio 4) 3.2% 3.3% 3.2% 3.4% 3.4% Electric power generation by partial oxidation retorming method (reforming temperature 1100° C.) Electric energy kJ/fuel kg initial performance 16200 15590 15730 14020 14420 100 hours later 16190 15590 15720 13910 14220 performance 100 hours later 0.06% 0.00% 0.06% 0.78% 1.39% deterioration ratio Thermal efficiency 2) initial performance 35% 34% 34% 30% 32% CO_(2 generation) kg/fuel kg initial performance 2.992 3.028 3.019 3.101 3.077 Energy per CO₂ KJ/CO₂-kg initial performance 5414 5149 5210 4521 4686 Preheating energy 3) kJ/fuel kg 1740 1750 1740 1630 1670 Preheating energy ratio 4) 10.7% 11.2% 11.1% 11.6% 11.6%

INDUSTRIAL APPLICABILITY

[0074] As described above, a fuel of the invention containing hydrocarbon compounds with specific compositions has performances with small deterioration by using in a fuel cell system and can provide high output of electric energy and other than that, the fuel can satisfy a variety of performances for a fuel cell system. 

1. A fuel for use in a fuel cell system, wherein said fuel comprises 60 mol. % or more of saturates, 40 mol. % or less of olefins, 0.5 mol. % or less of butadiene, 0.1 mol. % or more of isoparaffin in saturates having carbon atoms of 4 or more, and is a gaseous phase under normal temperature and pressure.
 2. A fuel according to claim 1, wherein a sulfur content is 50 ppm by mass or less.
 3. A fuel according to claim 1, wherein a content of hydrocarbons having carbon numbers of 2 or less is 5 mol. % or less, a content of hydrocarbons having carbon numbers of 3 and 4 in total is 90 mol. % or more and a content of hydrocarbons having carbon numbers of 5 or more is 5 mol. % or less.
 4. A fuel according to claim 2, wherein a content of hydrocarbons having carbon numbers of 2 or less is 5 mol. % or less, a content of hydrocarbons having carbon numbers of 3 and 4 in total is 90 mol. % or more and a content of hydrocarbons having carbon numbers of 5 or more is 5 mol. % or less.
 5. A fuel according to claim 4, wherein vapor pressure at 40° C. is 1.55 MPa or less.
 6. A fuel according to claim 4, wherein density at 15° C. is 0.500 to 0.620 g/cm³.
 7. A fuel according to claim 6, wherein vapor pressure at 40° C. is 1.55 MPa or less.
 8. A fuel according to claim 4, wherein corrosiveness to copper at 40° C. for 1 hour is 1 or less.
 9. A fuel according to claim 5, wherein corrosiveness to copper at 40° C. for 1 hour is 1 or less.
 10. A fuel according to claim 6, wherein corrosiveness to copper at 40° C. for 1 hour is 1 or less.
 11. A fuel according to claim 7, wherein corrosiveness to copper at 40° C. for 1 hour is 1 or less.
 12. A fuel according to claim 4, wherein heat capacity of the fuel is 1.7 kJ/kg ° C. or less at 15° C. in gaseous phase.
 13. A fuel according to claim 7, wherein heat capacity of the fuel is 1.7 kJ/kg ° C. or less at 15° C. in gaseous phase.
 14. A fuel according to claim 9, wherein heat capacity of the fuel is 1.7 kJ/kg ° C. or less at 15° C. in gaseous phase.
 15. A fuel according to claim 10, wherein heat capacity of the fuel is 1.7 kJ/kg ° C. or less at 15° C. in gaseous phase.
 16. A fuel according to claim 11, wherein heat capacity of the fuel is 1.7 kJ/kg ° C. or less at 15° C. in gaseous phase. 